The transfer of cargo from a vessel towards an offshore construction in general takes place with a hoisting crane. The cargo then for example is placed upon a pallet or inside a crate which hangs down at hoisting cables and needs to be put down at a loading platform which is positioned at a considerable height above the water level, in particular above the maximum expected wave height. If there is no crane on the offshore platform itself, then a hoisting crane of the vessel needs to be present. Such a vessel crane in general is a special (heavy) crane which is preferably capable of compensating vessel movements to a certain degree. The vessel movements in relation to weather conditions however still limit the use of such cranes. Furthermore the pendulum effect of the hoisting cables limit the weight of the cargo which can be transferred. Also this type of cargo transfer can not be used towards unmanned offshore constructions.
The transfer of people from a vessel towards an offshore construction in general takes place via a gangway as bridging element placed in between them. For example EP-1 315 651 shows a transfer system for positioning a telescopingly extendable gangway with a fixed and a movable gangway section between a vessel and an offshore construction. A hydraulic piston extends between the fixed and movable gangway sections and is able to adjust the total length of the gangway. The fixed gangway section is pivotally connected, around a horizontal rotation axis, to an upper end of an upright supporting column by means of a hinge connection. A hydraulic piston extends between the supporting column and the fixed gangway section and is able to adjust the angle of inclination of the gangway. The supporting column itself is rotatably mounted, around a vertical rotation axis, on the deck of the vessel by means of a turntable. Hydraulics is provided for adjusting the rotational position of the assembly of the column and gangway around the turntable relative to the vessel. The movable gangway section at its free end is provided with hydraulically operable coupling claws which are designed to be coupled to a substantially vertical gripping bar which is mounted to the offshore construction.
During coupling the vessel is sailed and maneuvered to a suitable starting position relative to the offshore construction. Subsequently the gangway by means of suitable steering of the respective hydraulics is swiveled/rotated such that it is directed towards a center part of the gripping bar. Then the gangway is extended until the coupling device is able to enclose the gripping bar. At that moment the coupling claws are moved from an open position towards a closed position. The closing is such that the claws still have the freedom to slide upwards along the gripping bar. As soon as an aimed coupling position along an upper part of the gripping bar just beneath a loading platform is reached, the coupling claws are moved further towards each other in order to fixedly clamp the gripping bar there. Finally, all the hydraulics for positioning the gangway are set free, such that the gangway is freely able to extend or retract, change its angle of inclination around the horizontal rotation axis, and/or change its rotational turntable position around the vertical rotation axis. The vessel then at the same time is kept in place by means of a suitable driving of thrusters or the like in dependency of satellite navigation. Thus no big forces are exerted on the gangway and the gripping bar.
A disadvantage with this known transfer system is that it is less suitable to transfer heavy cargo loads of for example 2 tons or more from the vessel towards the offshore construction and vice versa. In particular during heavy weather conditions this could easily result in too big forces occurring on the gangway and supporting column. This in turn would require the gangway and supporting column to be constructed relative strong and heavy. Furthermore a coupling under heavy weather conditions may be somewhat difficult. Forces which occur at the time that the gangway hits the coupling bar are difficult to manage under such conditions.